1. Field of the Invention
The present invention relates to ultrasonic imaging, such as ultrasound imaging, and more particularly to such imaging which scans an object with multiple ultrasonic beams which are simultaneously transmitted.
2. Background of the Invention
During the last two decades, advances in signal processing and electronics have helped ultrasound imaging to become one of the major tools in non-invasive clinical diagnosis. By transmitting a series of high frequency pulses, ultrasound imaging allows the examination of internal organs with far less risk than conventional surgical techniques or X-rays. Ultrasound imaging is used in measuring the performance of the heart, the flow of blood, to identify tumors, and in prenatal care and diagnosis.
A simplified block diagram of a typical prior art ultrasound imaging system is indicated generally at 10 in FIG. 1. A piezoelectric transducer array 12, comprising a stationary array of many small transducers, is stimulated by a high frequency transmitter circuit 14. When array 12 is so stimulated, it generates an ultrasonic pulse in a relatively narrow beam 16, such being illustrated schematically. In FIG. 1, transducer array 12 is substantially flush against and aimed into an object 18 which, in the case of medical ultrasound imaging, comprises a human body.
After transmission of beam 16, the reflections of the pulse from body 18 are detected by array 12 with such signals being processed by a receiver 20 for display on a cathode ray tube 22.
A two-dimensional image can be obtained by sweeping a beam, like beam 16, through a sector. For example, in a phased-array ultrasound system such as system 10, stationary array 12 is electronically controlled to steer and focus such a beam. Such an array is illustrated in R. D. Gatzke, J. T. Fearnside, and S. M. Karp, "Electronic Scanner for a Phased-Array Ultrasound Transducer," Hewlett-Packard Journal, pp. 13-20, Dec. 1983, which is incorporated herein by reference.
Current pulse echo imaging systems gather data sequentially. In a phased-array imaging system, like system 10, a pulse is transmitted in a narrow beam with the focus optimized for one line. Using dynamic receive focusing, the pulse is traced as it travels through the body. This process is repeated for each line until a complete image, 100-200 lines, is acquired. The transmission rate is selected so that echos of the transmitted pulse have time to return from the deepest target before the next pulse is transmitted.
Real-time systems must image all points at a rate of at least 30 frames per second. If T denotes the transmission period, then for a typical depth of image of 20 cm, a speed of sound of 1450 m/sec, and a CRT frame rate of 30 frames/sec, the best available line resolution is ##EQU1##
From (1.1), the major limitation in resolution and acquisition rates is the speed of sound in the body. This limitation leads to compromises in the design of present equipment, and limits the capabilities of future ultrasonic scanners.
A recent development of great interest is three-dimensional (3-D) acoustical imaging, where a stacked set of 2-D scans yields a 3-D image of an internal organ. A three-dimensional array is described in H. A. McCann et al., "Multidimensional Ultrasonic Imaging for Cardiology," Proc. of the IEEE, Vol. 76, No. 9, Sep. 1988, which is incorporated herein by reference. For example, such a technique could allow a non-invasive view of the heart arteries for the efficient diagnosis of heart diseases. However, current techniques and acquisition rates make impossible the implementation of a 3-D imaging system operating in real time.
One approach to overcome this problem is to apply massively parallel processing in the circuitry that forms the received echoes. Such a scheme is disclosed in J. Poulton, 0. Von Ramm, and S. Smith, "Integrated circuits for 3-D Medical Ultrasound Imaging," MCNC Technical Bulletin, Vol. 3, No. 4, Jul./Aug. 1987, which is incorporated herein by reference. For each transmitting beam, which illuminates many points simultaneously, 64 simultaneous receiving beams are formed over an 8.times.8 line grid. Parallel processing then can be applied for the simultaneous processing of all received signals. Such simultaneous parallel processing is disclosed in M. O'Donnell, "Applications of VLSI Circuits to Medical Imaging," Proc. of the IEEE, Vol. 76, No. 9, pp. 1106-1114, Sep. 1988, which is incorporated herein by reference.
Turning now to FIG. 2, indicated generally at 24 is a two-beam ultrasound system for creating an image in a single plane. Included therein are a total of K transducer elements, three of which are labeled elements 1, 2, K. Each element is associated with two time delays, d.sub.11 through d.sub.K2, where transducer element 1 is connected to delays d.sub.11, d.sub.12 ; transducer element 2 is connected to delays d.sub.21, d.sub.22 ; and transducer element K is connected to delays d.sub.K2, d.sub.K2.
A commercially available pulse generator 26 is connected to each of the delays via conductor 28. When pulse generator 26 produces a pulse on conductor 28 it is applied to each of the delays. After a predetermined time delay, the pulse is applied to the transducer to which each delay is connected. The transducer elements generate an ultrasonic pulse which propagates into an object against which the transducer array is held.
The length of the delay imposed on a pulse applied thereto for each of time delays d.sub.11 through d.sub.K2 is variable and is controlled by a computer in a known manner. Through such computer control, a preselected number of ultrasonic beams are caused to propagate from the center of the array with each beam having a preselected angle thereto. Appropriate choices of the delay settings are used to select both the angle and focus length of a transmitted beam.
Reception of beam reflections operates in a reverse manner. When an ultrasonic reflection strikes one of transducer elements 1-K, the element generates an electrical signal proportional to the ultrasonic reflection which is applied to the time delays. This delays each signal by a preselected value in the same fashion as the transmitted pulse is delayed. Each of the delayed received signals is applied to a conductor, like the signal from delay d.sub.11 is applied to conductor 30 and the signal from delay d.sub.12 is applied to conductor 32. The signals from one of the delays associated with each transducer element are summed by a conventional summing device 34 with the signals from the other time delay associated with each transducer element being summed at a separate summing device 36.
Selection of the reception angle and focus is accomplished in the same manner as selection of transmission angle and focus, i.e., by setting different time delays in a known manner. The array is thus caused to focus along each transmission axis at progressively increasing distances thereby sensing the ultrasonic reflections generated by each transmitted pulse.
System 24 comprises a 2-beam system, i.e., two ultrasonic pulses are simultaneously transmitted into the object. The time delays associated with summing device 34 are used to control the angle and focusing of one of the beams in the reception mode while the delays connected to summing device 36 are used to so control the other beam. This approach is a direct extension of the single-beam system described in R. D. Gatzke, J. T. Fearnside, and S. M. Karp, "Electronic Scanner for a Phased-Array Ultrasound Transducer," Hewlett-Packard Journal, pp. 13-20, Dec. 1983, and in its general form requires K.times.M delay elements for a system having M beams and K transducer elements. It should be appreciated that alternative beam-forming techniques which require a reduced number of delay elements could be equally well used to provide a multi-beam ultrasound imaging system. For example see J. Butler, R. Lowe, "Beam-Forming Matrix Simplifies Design of Electronically Scanned Arrays," Electronic Design, pp. 170-173, Apr. 12, 1961.
Consideration will now be given to the problem of interbeam interference which arises with a multi-beam ultrasonic system such as that depicted in FIG. 2. Assume the system has M beams and a transducer array of K elements. Suppose that only beam j is active and there is a single target at distance D and angle .THETA. from the array (FIG. 3). Assuming the media is free of frequency distortion and attenuation, the value of the ultrasound signal that illuminates the target is then given by ##EQU2## where d.sub.kj is the delay required at the input of the k-th element for the electronic focusing of beam j (see FIG. 2), .delta..sub.kD is the time required for the signal generated by element k to reach the target, and G(t) is the signal generated by the pulse generator. The superscript s denotes that the system is in single-beam mode.
From FIG. 3, .delta..sub.kD =l.sub.kD /V where V is the speed of sound and l.sub.kD is the distance of element k from the target. By applying the Pythagorean theorem, it can be shown that ##EQU3## where e(k)=1/2(2k-K-1), is the distance of element k from the center of the array and .tau. is the distance between the elements of the transducer. For beam focusing at a distance D.sub.fj, it is required that d.sub.kj +.delta..sub.kD.sbsb.fj =constant=T.sub.f. Then from (2.2): ##EQU4## where D.sub.fj is the focusing distance of beam j, .THETA..sub.j is the steering angle of beam j, and T.sub.f is a constant selected so that d.sub.kj is always positive.
Similarly to (2.1), the received echo from beam j is given by ##EQU5## where .alpha. denotes the target reflectivity.
When all M beams are transmitted at the same time, if T.sup.m (t) denotes the composite signal that illuminates the target in the multi-beam mode, then: ##EQU6## From (2.4) and (2.5), the received signal at the output of the receiver for the j-th beam is given by ##EQU7## From (2.1)-(2.6) it can be shown that: ##EQU8## The second term in (2.7) implies that the signal received from an image target through some beam j will be affected by the transmitted energy of all other beams. This term is referred to herein as interbeam interference and is mainly due to the sidelobe energy of each beam. The interference occurs in two dimensions, namely along each arc which crosses the beams and along the axis of each beam. In a real imaging system, this interference is seen as a "butterfly pattern" around the true target.
Although prior art systems have processed data collected by multiple beam (and single-beam) ultrasonic imaging systems, they have either a very high degree of complexity or do not minimize the effect of interbeam interference to the extent that the present invention does.